Platinum group metals such as platinum, palladium and rhodium are used in a lot of electric appliances, in addition to being used as industrial catalysts and automobile exhaust gas purifying catalysts. Although these platinum group metals are expensive and have been recycled heretofore by being recovered after use, it cannot be said that the techniques are satisfactory. Recently, the importance of recycling has increased furthermore by taking resource conservation into consideration.
Heretofore, a lot of methods such as a precipitation method (Patent Document 1), an ion exchange method (Patent Document 2), an electrolytic deposition method (Patent Document 3) and a solvent extraction method have been proposed for recovery of platinum group metals and have been carried out. Among these methods, a solvent extraction method has been widely employed because of its economical efficiency and operability.
However, since an effective separation reagent for rhodium does not exist in the separation of platinum group metals using the solvent extraction method, rhodium has been recovered from the separation residue after being separated from other metals (Patent Documents 4 to 6). However, it is inefficient from an economical point of view to leave rhodium, which is the most expensive of the platinum group metals, until the end of the process.
Selective extraction of palladium with a sulfur-containing organic compound, dialkyl sulfide (DAS), has widely been used for mutual separation of palladium and platinum by the solvent extraction method (Patent Document 7). Among DAS, dihexyl sulfide (DHS) is commonly used but is insufficient in an extraction %, and it is pointed out that the sulfide groups are likely to be oxidized.
The present inventors have found that it is possible to extract palladium within a short time when compared with the case of using DHS which has been used heretofore, and to separate other platinum group metals and a base metal, and also to separate and recover palladium with high efficiency by using a sulfur-containing diamide compound such as thiodiglycolamide, 3,3′-thiodipropionamide or 3,6-dithiaoctanediamide in place of conventional DHS as an extractant of palladium (Patent Document 8). However, rhodium cannot be extracted by the extractants.
Patent Document 1:
Japanese Unexamined Patent Application, First Publication No. Hei 10-102156
Patent Document 2:
Japanese Unexamined Patent Application, First Publication No. Hei 7-310129
Patent Document 3:
Japanese Unexamined Patent Application, First Publication No. Hei 6-300854
Patent Document 4:
Japanese Examined Patent Application, Second Publication No. Hei 1-30896
Patent Document 5:
Japanese Unexamined Patent Application, First Publication No. Hei 8-193228
Patent Document 6:
Japanese Unexamined Patent Application, First Publication No. 2004-332041
Patent Document 7:
Japanese Unexamined Patent Application, First Publication No. Hei 9-279264
Patent Document 8:
International Publication No. WO 2005/083131 pamphlet